Time of flight (TOF) mass spectrometers are widely used to determine the mass to charge ratio of charged particles on the basis of their flight time along a path. The charged particles, usually ions, are emitted from a pulsed source in the form of a packet, and are directed along a prescribed flight path through an evacuated space to impinge upon or pass through a detector. In its simplest form, the path follows a straight line and in this case ions leaving the source with a constant kinetic energy reach the detector after a time which depends upon their mass, more massive ions being slower. The difference in flight times between ions of different mass depends upon the length of the flight path, amongst other things; longer flight paths increasing the time difference, which leads to an increase in mass resolution. When high mass resolution is required it is therefore desirable to increase the flight path length. However, increases in a simple linear path length lead to an enlarged instrument size, increasing manufacturing cost and requiring more laboratory space to house the instrument.
Various solutions have been proposed to increase the path length whilst maintaining a practical instrument size, by utilising more complex flight paths. Many examples of charged particle mirrors or reflectors have been described, as have electric and magnetic sectors, some examples of which are given by H. Wollnik and M. Przewloka in the Journal of Mass Spectrometry and Ion Processes, 96 (1990) 267-274, and G. Weiss in U.S. Pat. No. 6,828,553. In some cases two opposing reflectors or mirrors direct charged particles repeatedly back and forth between the reflectors or mirrors; offset reflectors or mirrors cause ions to follow a folded path; sectors direct ions around in a ring or a figure of “8” racetrack. Herein the terms reflector and mirror are used interchangeably. Many such configurations have been studied and will be known to those skilled in the art.
There are essentially two possible types of flight path: an open flight path and a closed flight path. In an open flight path, the ions do not follow a repeated path and as a result, in an open flight path ions of different mass to charge ratio therefore can never overlap whilst travelling in the same direction upon the same flight path. However, in a closed flight path, the ions do follow a repeated path and return to the same point in the flight path after a given time, to proceed upon the flight path once again, whereby ions of different mass to charge may overlap whilst following the same path. A particular advantage of having an open flight path, e.g. the simple linear flight path, is the theoretically unlimited mass range able to be analysed from each ion packet emitted from the pulsed source. In the case of a closed flight path, e.g. as in directly opposing mirror time of flight instruments, and all designs in which ions repeatedly follow a given flight path, this advantage is lost as, during the flight, the packet becomes a train of packets of different mass to charge particles, the length of which train increases during the flight time. On increasing the flight time, the front of this train of packets may eventually fold around and catch up with the rear on the repeated path, packets of different mass to charge particles then arriving at the detector at the same time. Detection in such a case would yield an overlapping mass spectrum, which would require some form of deconvolution. This has led in practice to a reduced mass range, or a limit on the length of the flight path that can be utilised, or both, in analysers of this type. To avoid this, it is desirable to retain the unlimited mass range available from time of flight instruments that utilise an open or non-repeated path. However, reflecting time of flight geometries that produce a folded path and multiple sector designs have the disadvantage that they require multiple high-tolerance ion optical components, adding cost and complexity, as well as generally being larger in size.
In addition to these considerations, for high mass resolution it is important that charged particles of the same mass to charge ratio emitted from a finite volume within the pulsed source and having trajectories with varying angular divergence all reach the detector at the same time. This may be termed temporal focusing on initial angle and position. A relatively wide range of angular divergence (up to few degrees) and spatial spread (submillimeter to several tens of mm) should be accepted by the time of flight analyser, all particles accepted being brought to a time focus at the detector, which is to say, ions of the same mass to charge ratio arrive at the detector at the same time regardless of their initial angular divergence or spatial position at the source. For high resolution, reflectors and sectors that are utilised to increase the flight path length must be designed such that this temporal focusing is higher than to first order, preferably the focusing should be to third order or higher.
Still further to these considerations, time focusing of particles having different energies must also be achieved for high mass resolution. Energy spreads up to several tens of percent of the nominal beam energy might have to be accommodated for particles emitted by some types of pulsed ion source, requiring TOF analysers where the time of flight is energy independent to high order. A variety of designs has been proposed for both reflectors and sectors that have improved time focusing for particles of differing energies. Some reflectors having improved time focusing for particles of differing energies include grids to better control the electric field within the reflector, however such reflectors are less suitable for multi-reflection systems, as ions are lost through collisions with the grids at each reflection, and the overall transmission of the system after multiple reflections is compromised.
For reflectors, it has been noted that application of a linear electric reflection field, yielding harmonic charged particle motion, produces perfect time focusing for particles of varying energies. Examples have been proposed by W. S. Crane and A. P. Mills in Rev. Sci. Instrum. 56(9), 1723-1726 (1985), Y. Yoshida in U.S. Pat. No. 4,625,112 and U. Andersen et.al. in Rev. Sci. Instrum. 69(4) 1650-1660 (1998), and others. The linear field produces a force upon the charged particles which increases linearly with increasing distance into the reflector. Higher energy particles travel faster but also travel further into the reflection field and spend the same time within it as do lower energy particles. Such a linear field is formed with a parabolic electrical potential. Confusingly, many prior art publications refer to the field as parabolic rather than the potential; a parabolic field does not result in harmonic motion. Difficulties exist with the use of such parabolic potential reflectors, however, as they tend to produce strong divergence of ion beams in directions orthogonal to the axis of reflection. This makes 2 or more reflections in such mirrors simply impractical. The quality of focusing in such fields also degrades as longer field-free regions are introduced between an ion source and entrance to such a mirror.
For multiple reflection systems the angular divergence of the charged particle beam must be constrained to conserve high transmission. Spatial focusing in the plane perpendicular to the direction of time-of-flight separation requires the presence of a strong (usually accelerating) lens on the entrance to the mirror as well as a field-free drift space prior to the entrance to the mirror, such as is contemplated in GB2,080,021. The use of multiple reflectors or multiple sectors requires sophisticated design and high tolerance manufacturing for each of the several reflectors or sectors, resulting in increased complexity and cost, as well as typically a larger instrument size. The construction could be made simpler and easier to control if the mirrors were planar, as proposed in SU1,725,289. Divergence in the shift direction parallel to the mirror's extension could be limited by using periodic lenses as proposed by A. Verentchikov et.al. in U.S. Pat. No. 7,385,187. However, such lenses themselves cause beam aberrations unless they are quite weak and can limit the quality of the final time focus and hence limit mass resolution.
For all such systems, high focusing voltages are required to get high quality of spatial and temporal focusing. More importantly in practice, the substantial non-linearity of the reflecting field even near the turning points in all mirrors of this type drastically reduces the tolerance to space charge, as described in WO06129109.
L. N. Gall et.al. in SU1247973 proposed an alternative parabolic potential arrangement in which charged particles are reflected in a structure having two coaxial electrodes, particles travelling between the two, orbiting the inner electrode. The electric field between the electrodes has independent components in the directions of the longitudinal (Z) axis and the radial (r) axis, which is to say that the force on the charged particle in the longitudinal direction is independent of the radial position of the particle. The presence of concentric electrodes produces a logarithmic potential term in r, and a parabolic potential term is present in Z. However the single reflecting embodiment described by Gall et.al. has a limited flight path length. Gall et.al. provide no teaching on how such a field could be utilised in a multi-reflecting structure. A further single-reflecting example utilising this type of field, but using separate potentials applied to a ring structure, was also given by V.P. Ivanov et.al. in Proc. 4th Int. Seminar on the Manufacturing of Scientific Space Instruments, Frunze, 1990, IKI AN, Moscow, 1990, vol. 2, 65-69. Both these single reflecting TOF instruments have limited mass resolution, the latter demonstrating only a resolving power of 40. The main problem with these systems relates to the precise definition of the field, especially at the points of ion injection and ejection. This problem stems from the necessity to avoid any field-free drift spaces within such a system in order to have axial field strictly linear along the entire ion path.
There remains a need for a compact, high resolution, unlimited mass range TOF which embodies perfect or near perfect angular and time focusing characteristics with a minimum of high tolerance components.
A brief glossary of terms used herein for the invention is provided below for convenience; a fuller explanation of the terms is provided at relevant places elsewhere in the description.
Analyser electrical field (also termed herein analyser field): The electric field within the analyser volume between the inner and outer field-defining electrode systems of the mirrors, which is created by the application of potentials to the field-defining electrode systems. The main analyser field is the analyser field in which the charged particles move along the main flight path.
Analyser volume: The volume between the inner and outer field-defining electrode systems of the two mirrors. The analyser volume does not extend to any volume within the inner field-defining electrode system, nor to any volume outside the inner surface of the outer field-defining electrode system.
Angle of orbital motion: The angle subtended in the arcuate direction as the orbit progresses.
Arcuate direction: The angular direction around the longitudinal analyser axis z. FIG. 1 shows the respective directions of the analyser axis z, the radial direction r and the arcuate direction ø, which thus can be seen as cylindrical coordinates.
Arcuate focusing: Focusing of the charged particles in the arcuate direction so as to constrain their divergence in that direction.
Asymmetric mirrors: Opposing mirrors that differ either in their physical characteristics (size and/or shape for example) or in their electrical characteristics or both so as to produce asymmetric opposing electrical fields.
Beam: The train of charged particles or packets of charged particles some or all of which are to be separated.
Belt electrode assembly: A belt-shaped electrode assembly extending at least partially around the analyser axis z.
Charged particle accelerator: Any device that changes either the velocity of the charged particles, or their total kinetic energy either increasing it or decreasing it.
Charged particle deflectors: Any device that deflects the beam.
Detector: All components required to produce a measurable signal from an incoming charged particle beam.
Ejector: One or more components for ejecting the charged particles from the main flight path and optionally out of the analyser volume.
Equator, or equatorial position of the analyser: The mid-point between the two mirrors along the analyser axis z, i.e. the point of minimum absolute electrical field strength in the direction of the analyser axis z.
External ejection trajectory: The trajectory outside the analyser volume taken by the beam on ejection from the analyser.
External injection trajectory: The trajectory outside the analyser volume taken by the beam on injection into the analyser.
Field-defining electrode systems: Electrodes that, when electrically biased, generate, or contribute to the generation of, or inhibit distortion of the analyser field within the analyser volume.
Injector: One or more components for injecting the charged particles onto the main flight path through the analyser.
Internal ejection trajectory: The trajectory inside the analyser volume taken by the beam on ejection from the main flight path.
Internal injection trajectory: The trajectory inside the analyser volume taken by the beam on injection prior to joining the main flight path.
Main flight path: The stable trajectory that is followed by the charged particles for the majority of the time that the particles are being separated. The main flight path is followed predominantly under the influence of the main analyser field.
m/z: Mass to charge ratio
Offset lens embodiments: Embodiments in which the arcuate focusing lenses are displaced from the equatorial position of the analyser.
Principal beam: the beam path taken by ions having the nominal beam energy and no beam divergence.
Receiver: Any charged particle device that forms all or part of a detector or device for further processing of the charged particles.